Alteration in neuroendocrine function is a major cause of reproductive aging processes in females. Reduced sensitivity to estrogen appears to play an important role in this dysfunction. Chronic exposure to estrogen and possibly other ovarian secretions is in turn implicated in the etiology of reduced estrogenic sensitivity, since long-term withdrawal from ovarian hormones during adult life lessens these age-related reductions in sensitivity. This proposal aims to elucidate the molecular basis for these estrogen dependent aging processes in the hypothalamus-preoptic area (HPOA) of the female C57BL/6J mouse, a well characterized model of female reproductive aging. Three major questions are addressed. First, studies are proposed to elucidate the molecular and cellular basis for reduced concentrations and altered intracellular dynamics of estrogen receptor during aging. The role of altered expression of the gene encoding the estrogen receptor in its age-related reduction and altered intracellular dynamics will be determined using a solution hybridization/RNAse protection assay for mouse estrogen receptor mRNA. The relative contributions of reduced intraneuronal concentrations of the estrogen receptor versus loss of neurons that actively produce estrogen receptor will be assessed. These studies will use immunocytochemistry and in situ hybridization to map the age-related changes in the distribution and intraneuronal density of estrogen receptor and estrogen receptor mRNA. The second major question concerns the effect of aging on estrogen-modulated gene expression in the hypothalamus-preoptic area. Because estrogen receptor levels are only partially reduced in brain and because of the prevalence of selective age related changes, the hypothesis will be tested that genes responsive to estrogen exhibit variable impairment in responsiveness during aging: specifically, genes most sensitive to estrogenic modulation show the least age-related reduction in sensitivity to estrogen; whereas less sensitive genes show greater impairment. These studies will involve exposing young and old mice to variable concentrations of circulating estradiol and measuring the relationship between plasma estradiol, cell nuclear occupancy of the estrogen receptor and level of induction (or suppression) of the estrogen modulated mRNA. Finally, studies will determine whether the age- related alterations in estrogen receptor parameters and in estrogen- responsive genes that are identified in the aforementioned studies are attenuated by long-term ovariectomy, and thus may mediate some of the protective effects of hormonal withdrawal on reproductive aging processes. The proposed studies should contribute to understanding the molecular mechanism and cellular targets for altered responsiveness to estrogen in the aging brain. In addition to advancing understanding of an important reproductive aging process, this knowledge has implications for post- menopausal hormone withdrawal and for estrogen replacement therapy, the most common steroid hormone replacement regimen in women.